Impact of Nitrogen Fertiliser Usage in Agriculture on Water Quality

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

• Manuscript ID pollutants-3599575

   

   

  Brief summary:

  This article explores the environmental consequences of synthetic nitrogen fertilizer use in agriculture, with a particular focus on its impact on water quality. It aims to quantify the link between fertilizer use and nitrate contamination in water bodies through a systematic review and meta-analysis. Addressing this requires targeted agricultural practices, better data reporting standards, and integrated environmental policy responses. One of the findings is that the excessive nitrate in water causes eutrophication, harmful algal blooms, and health risks such as methemoglobinemia and possible cancer risks.

  Comments on the article:

  • The manuscript demonstrates a commendable level of technical writing and knowledge in the subject.
  • The abstract and introduction are well-structured, providing clarity on the scope and objectives of the study.
  • There is Logical flow and good paragraph transitions in most sections.
  • The inclusion of Table 1 summarizing the nitrate data across 18 studies is very helpful and interesting.
  • Captions are too short. For example, 'Figure 2: Distribution of true effects' could be 'Distribution of true effects for nitrate concentration across 18 included studies using a random-effects model.'
  • It is recommended to expand literature linkage in the Discussion to better engage with existing research.
  • The manuscript is well-cited with over 60 references from high-quality, peer-reviewed journals.
  • Most of the references are recent (2020-2024), ensuring the study is grounded in current research.
  • Some citations (like [10] and [41]) refer to online websites (Yara UK, StatisticsHowTo) that are not peer-reviewed. Where possible, it is recommended to replace with peer-reviewed or institutional technical reports.

   

  Final recommendation: Accept after minor revisions.

   

   

Author Response

Reviewer 1

We sincerely thank the reviewer for the thoughtful and constructive feedback provided. We are grateful for the positive remarks regarding the quality and structure of the manuscript, and we appreciate the recommendation for acceptance. All suggestions for improvement have been carefully considered and addressed in detail below.

Reviewer comment: Captions are too short. For example, 'Figure 2: Distribution of true effects' could be 'Distribution of true effects for nitrate concentration across 18 included studies using a random-effects model.

Response: Thank you for your constructive feedback regarding the figure captions. We agree that clearer and more detailed captions enhance the readability and interpretation of the figures. In response, we have carefully revised all figure captions to be more descriptive and informative.

Reviewer comment: Some citations (like [10] and [41]) refer to online websites (Yara UK, StatisticsHowTo) that are not peer-reviewed. Where possible, it is recommended to replace with peer-reviewed or institutional technical reports.

Response: We appreciate the reviewer’s observation regarding the use of non-peer-reviewed sources in References [10] and [41]. In response, we have replaced Reference [10] (Yara UK) with a more appropriate institutional source: (Tyldesley, 1974), published by the University of Oxford. This source offers a clear and authoritative explanation of the ammonium nitrate production process and aligns with the content originally cited.

Regarding Reference [41] (StatisticsHowTo), we conducted an extensive search for a peer-reviewed or institutional alternative presenting the combined mean formula. However, we were unable to identify a more suitable source that clearly and accessibly illustrates this standard statistical method. The formula used is widely accepted and non-controversial, and the cited website presents it accurately. Given this, we have retained the original reference, limiting its use strictly to describing the basic mathematical procedure. If preferred, we are happy to relocate this content to the supplementary material or acknowledge it in a footnote.

Reference:
Tyldesley, A. (1974). Ammonium Nitrate, a simple fertiliser, or dangerous explosive? [online] Available at: https://www.chem.ox.ac.uk/files/ks4ammoniumnitratepdf.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript titled “Impact of Nitrogen Fertiliser Usage in Agriculture on Water Quality” explores a relevant and significant topic. While the subject matter presented are of interest, the manuscript requires revision. Few issues must be addressed to improve the clarity, structure, and scientific rigor of the paper, as outlined in the detailed comments below.

1. Line 19: Based on the reported average value of 34.283 mg/L, which demonstrates a significant impact of nitrogen fertilizers on water quality, the authors should mention the WHO or regulatory nitrate concentration thresholds for clearer interpretation.
2. Line 56: (𝑵𝟐) reacts with hydrogen (𝑯𝟐) to form ammonia (𝑵𝑯𝟑−). Authors should check the formular font size, and correct accordingly.
3. Lines 93–104: There are repeated ideas on research gaps and contributions; the authors should consolidate these to avoid redundancy.
4. Line 90-92: The study in India’s Indo-Gangetic Plains (IGP) region found that 27% of children, 19% of men, and 16% of women may be affected by nitrate contamination, with agriculture identified as the primary source. Does this refer to clinical nitrate poisoning or exposure levels? The authors should clarify this in the text.
5. Please carefully check abbreviations (e.g., harmful algal blooms (HABs)). The authors are advised to either define all abbreviations upon their first occurrence or include an abbreviation table to enhance clarity. Check for similar issues throughout the manuscript.
6. Figure 3. The meta-analysis of nitrate concentration in water bodies combines different studies. Did you interpolate these studies from different sources or extrapolate from a single source? If a single source was used, please reference it accordingly.
7. Based on authors Figure 7. what do the distinct color differences represent?
8. Line 287-288: Are the 18 studies from similar geographic contexts? 
9. Line 395-396: State the limit thresholds and compare them to your findings
10. Ensure this comparison is also included in the conclusion section for consistency, and to quantitatively improve it.

Author Response

Reviewer 2

We thank the reviewer  for the careful evaluation of our manuscript and for recognising the relevance and significance of the topic. We appreciate the reviewer’s constructive feedback and have revised the manuscript accordingly to improve its clarity, structure, and scientific rigour. Detailed responses to each specific comment are provided below.

Reviewer Comment: Line 19: Based on the reported average value of 34.283 mg/L, which demonstrates a significant impact of nitrogen fertilizers on water quality, the authors should mention the WHO or regulatory nitrate concentration thresholds for clearer interpretation.

Response: Thank you for this helpful suggestion. We have revised the sentence to include relevant regulatory thresholds for nitrate concentrations, thereby providing a clearer context for interpretation.

Reviewer Comment: Line 56: () reacts with hydrogen () to form ammonia (). Authors should check the formular font size, and correct accordingly.

Response: Thank you for pointing this out. The formula font size and formatting have been corrected to ensure consistency and clarity in the revised manuscript.

Reviewer Comment: Lines 93–104: There are repeated ideas on research gaps and contributions; the authors should consolidate these to avoid redundancy.

Response: Thank you for your observation. We have revised this section to consolidate the discussion of research gaps and contributions, thereby eliminating redundancy and improving clarity.

Reviewer Comment: Line 90-92: The study in India’s Indo-Gangetic Plains (IGP) region found that 27% of children, 19% of men, and 16% of women may be affected by nitrate contamination, with agriculture identified as the primary source. Does this refer to clinical nitrate poisoning or exposure levels? The authors should clarify this in the text.

Response: Thank you for this helpful clarification request. We have amended the sentence to specify that the figures refer to elevated nitrate exposure levels, rather than confirmed clinical cases of nitrate poisoning.

Reviewer Comment: Please carefully check abbreviations (e.g., harmful algal blooms (HABs)). The authors are advised to either define all abbreviations upon their first occurrence or include an abbreviation table to enhance clarity. Check for similar issues throughout the manuscript

Response: Thank you for this valuable suggestion. We have carefully reviewed the manuscript to ensure that all abbreviations, including “harmful algal blooms (HABs),” are clearly defined upon first use.

Reviewer Comment: Figure 3. The meta-analysis of nitrate concentration in water bodies combines different studies. Did you interpolate these studies from different sources or extrapolate from a single source? If a single source was used, please reference it accordingly.

Response: Thank you for your comment. We confirm that the meta-analysis presented in Figure 3 is based on data synthesised from multiple independent studies, not extrapolated from a single source. The contributing studies are clearly listed in Figure 3 and referenced accordingly in the manuscript.

Reviewer Comment: Based on authors Figure 7. what do the distinct color differences represent

Response: Thank you for your helpful comment. We have updated the caption for Figure 7 to explain the colour differences more clearly.

Reviewer Comment: Line 287-288: Are the 18 studies from similar geographic contexts?

Response: Thank you for your comment. We would like to note that this point is addressed in Section 3.1 of the Results, where we state that the final meta-analysis included 18 studies representing a range of geographical locations, thereby justifying the use of a random-effects model. We believe this adequately clarifies the geographic diversity of the included studies.

Reviewer Comment: Line 395-396: State the limit thresholds and compare them to your findings.

Response: Thank you for this helpful suggestion. We have revised the text to explicitly state the regulatory threshold limits and compare them with our findings.

Reviewer Comment: Ensure this comparison is also included in the conclusion section for consistency, and to quantitatively improve it

Response: We have revised the conclusion section to include a direct comparison of the observed nitrate levels with relevant regulatory thresholds.

Reviewer 3 Report

Comments and Suggestions for Authors

1. The introduction gives a general overview of nitrogen fertiliser use and its environmental impacts, which is fine, but it doesn't really explain why this particular review was necessary. The topic — nitrogen fertiliser and water quality — has been covered extensively in the literature, so the authors need to do more to convince the reader that this paper brings something new to the table. At the moment, it's not clear what the novelty or specific focus of the study is. Is it the timeframe? The data sources? The geographical coverage? The methodological approach? This needs to be stated up front. Otherwise, the introduction feels like it's just repeating well-known background without giving a clear reason for this meta-analysis. A review article, especially on such a broad topic, really needs a sharper justification — what gap is being filled, and why now?
2. I recommend numbering the formulas using Arabic numerals. Additionally, the space between the formula and the number is currently filled with dots—  remove the dots or replace them with an appropriate amount of white space.
3. Figure 3 is unclear, difficult to read, and quite confusing in its current form. I strongly suggest reorganizing the image to improve its legibility and overall presentation.
4. Considering that this is a review article,  the discussion section should be rather limited in scope and depth. The current version reads more like an extended summary of results, rather than a critical and reflective synthesis of the literature. Given the complexity and importance of the topic, a more in-depth discussion is warranted.
5. The conclusion is clear, but I find it somewhat too general for a meta-analysis. It restates the key findings, but doesn’t fully convey the weight or implications of the results. Given the relevance of the topic, I believe the authors could strengthen this section by being more specific — particularly regarding what should be done in response to the findings.

Author Response

Reviewer 3

Reviewer Comment: The introduction gives a general overview of nitrogen fertiliser use and its environmental impacts, which is fine, but it doesn't really explain why this particular review was necessary. The topic — nitrogen fertiliser and water quality — has been covered extensively in the literature, so the authors need to do more to convince the reader that this paper brings something new to the table. At the moment, it's not clear what the novelty or specific focus of the study is. Is it the timeframe? The data sources? The geographical coverage? The methodological approach? This needs to be stated up front. Otherwise, the introduction feels like it's just repeating well-known background without giving a clear reason for this meta-analysis. A review article, especially on such a broad topic, really needs a sharper justification — what gap is being filled, and why now?

Response: Thank you for this thoughtful and constructive comment. We agree that the original introduction did not sufficiently highlight the novelty or specific focus of the study. In response, we have revised the introduction to provide a clearer justification for the review.

Reviewer Comment: I recommend numbering the formulas using Arabic numerals. Additionally, the space between the formula and the number is currently filled with dots—  remove the dots or replace them with an appropriate amount of white space

Response: Thank you for this helpful formatting suggestion. We have revised the manuscript to number all formulae using Arabic numerals for clarity and consistency. In addition, the dotted spacing between each formula and its corresponding number has been removed and replaced with appropriate white space, in line with standard formatting conventions.

Reviewer Comment: Figure 3 is unclear, difficult to read, and quite confusing in its current form. I strongly suggest reorganizing the image to improve its legibility and overall presentation.

Response: Thank you for this valuable feedback. In response, Figure 3 has been substantially revised to improve its clarity and overall presentation. The updated version includes a clean white background, larger fonts, and clearer axis labelling. Confidence intervals for each study are clearly displayed, and Z-values have been added in bold, aligned on the right-hand side of the figure for better readability.

Reviewer Comment: Considering that this is a review article,  the discussion section should be rather limited in scope and depth. The current version reads more like an extended summary of results, rather than a critical and reflective synthesis of the literature. Given the complexity and importance of the topic, a more in-depth discussion is warranted.

Response: Thank you for this insightful observation. We agree that the original Discussion section leaned too heavily on summarising results, rather than critically engaging with the literature. In response, we have substantially revised this section to provide a more reflective and in-depth synthesis. The updated discussion now better contextualises the findings within existing research, highlights points of agreement and divergence, and addresses potential explanations for observed patterns in nitrate contamination.

Reviewer Comment: The conclusion is clear, but I find it somewhat too general for a meta-analysis. It restates the key findings, but doesn’t fully convey the weight or implications of the results. Given the relevance of the topic, I believe the authors could strengthen this section by being more specific — particularly regarding what should be done in response to the findings.

Response: Thank you for your valuable feedback. We acknowledge that the original conclusion was somewhat general and did not fully convey the implications of the findings. In response, we have revised the conclusion to be more specific and action-oriented.

Reviewer 4 Report

Comments and Suggestions for Authors

While the topic is timely, the methodological flaws (I² = 100%, no control group) currently limit the study’s contribution. Below are my comments for major revision.

1. The meta-analysis reports extreme heterogeneity (I² = 100%, Q-value = 11,453.208, *p*< 0.001), indicating that the included studies are too dissimilar to pool meaningfully. The prediction interval (13.014–55.552 mg/L) further confirms vast variability.
2. The pooled mean nitrate concentration (34.283 mg/L) lacks practical interpretability. Aggregating data from diverse ecosystems (e.g., groundwater vs. surface water), climates, and agricultural practices without subgroup analyses obscures context-specific insights.
3. The analysis lacks a control group (e.g., non-agricultural or pre-fertilization water quality data), relying solely on nitrate concentrations from fertilized sites. This design precludes causal attribution to fertilizers alone.
4. Funnel plot asymmetry suggests potential publication bias, yet the fail-safe N test (35,906 "missing" studies) is overrelied upon to dismiss bias. This test assumes missing studies are null-finding, which is unrealistic. It is recommended to supplement with Egger’s regression test for bias, then discuss how bias might affect results (e.g., small-study effects exaggerating pollution levels).
5. The discussion does not adequately address why nitrate levels vary widely (e.g., soil permeability, rainfall, fertilizer application timing). Regulatory thresholds (WHO/EU limits) are mentioned, but not contextualized against regional variability. I suggest expanding the "Understanding Heterogeneity" section (4.4) to link factors like soil type or climate to observed disparities. In addition, pooled nitrate levels should be compared to local standards (e.g., the EU’s 50 mg/L limit) to assess real-world risks.
6. On Table 1: Clarify units (e.g., "Mean (mg/L NO₃⁻)").
7. The Z-value (13.457) is misstated as "2-Value" in Figure 3. Correct typographical errors.
8. Figure 5 is cut off. Ensure all panels are visible.
9. Briefly cite evidence for precision agriculture’s efficacy (e.g., % reduction in leaching from CRFs or deep placement).
10. Specify which cancers/reproductive disorders are linked to nitrates (e.g., colorectal cancer) for clarity.
11. Simplify phrasing in entire manuscript (e.g., "synthesised using a single-group meta-analysis of means").
12. Appendix tables (A1–A3) are disorganized. Consolidate and clarify keyword iterations.

Comments on the Quality of English Language

N/A

Author Response

Reviewer 4

Reviewer Comment: “The meta-analysis reports extreme heterogeneity (I² = 100%, Q-value = 11,453.208, p < 0.001), indicating that the included studies are too dissimilar to pool meaningfully. The prediction interval (13.014–55.552 mg/L) further confirms vast variability.”

Response: We thank the reviewer for raising this important methodological concern. We acknowledge that the I² value of 100%, alongside the high Q-value and wide prediction interval, indicates substantial heterogeneity across the included studies. However, we respectfully argue that this degree of heterogeneity does not invalidate the pooling process, and instead reflects the complex ecological and methodological realities inherent in environmental meta-analyses involving nitrate pollution.

First, the use of a random-effects model, as adopted in our analysis, is specifically designed to accommodate such between-study variability. As recommended by Borenstein et al. (2010) and Viechtbauer (2010), random-effects models assume that true effects vary across studies due to differing environmental conditions, agricultural practices, sampling regimes, and geographies—which is precisely the case here. The goal, therefore, is not to estimate a single universal effect, but rather to capture the average effect across a distribution of true effects.

Second, the wide prediction interval (13.014–55.552 mg/L), rather than undermining the validity of the analysis, reinforces the need for context-specific nitrogen management policies. It reveals that while the average nitrate concentration is moderately elevated, some locations may face extreme contamination, while others may remain within acceptable limits. This insight has strong practical value for prioritising regionally tailored interventions and aligns with guidance from environmental meta-analysis frameworks.

Third, to ensure robustness despite the heterogeneity, we conducted sensitivity analyses (Figures 4 and 5), including the “one-study-removed” approach. These confirmed that no single study unduly influenced the pooled estimate, and that the overall direction and magnitude of the effect remained consistent across all iterations. Additionally, publication bias assessments (e.g., Duval & Tweedie’s trim-and-fill, Begg’s test) did not indicate systematic distortion in the pooled result.

Finally, as recommended by Fanelli and Ioannidis (2013), we interpret the pooled effect size in conjunction with heterogeneity metrics rather than in isolation. To reflect this, we have revised Section 4.4 of the Discussion to clearly explain the implications of heterogeneity and explicitly caution against overgeneralisation. We also now emphasise that while our average effect is meaningful, it should be interpreted as a benchmark rather than a universally generalisable value.

We hope this clarifies the rationale behind our approach and demonstrates that the heterogeneity observed is not only expected in environmental synthesis work but also analytically accounted for and substantively informative.

Reference:
Fanelli, D., & Ioannidis, J. P. (2013). US studies may overestimate effect sizes in softer research. Proceedings of the National Academy of Sciences, 110(37), 15031-15036.

Reviewer Comment:“The pooled mean nitrate concentration (34.283 mg/L) lacks practical interpretability. Aggregating data from diverse ecosystems (e.g., groundwater vs. surface water), climates, and agricultural practices without subgroup analyses obscures context-specific insights.”

Response: We thank the reviewer for highlighting this limitation, which we acknowledge and now address more explicitly in the revised manuscript. It is true that aggregating nitrate concentration data across diverse environmental contexts—such as groundwater and surface water, temperate and tropical climates, or varied fertilisation regimes—can dilute important localised dynamics. However, we respectfully argue that, even without formal subgroup analyses, the pooled mean of 34.283 mg/L remains a useful benchmark for several reasons:

1. Purpose of the meta-analysis: The goal of this review was to provide a broadly representative estimate of nitrate concentrations associated with nitrogen fertiliser use in agriculture, based on the most recent (2020–2024) empirical evidence. Given the lack of standardisation in reporting across studies (e.g., missing sample type, inconsistent use of control groups), subgroup meta-analyses were not feasible without compromising statistical robustness or introducing bias.
2. Heterogeneity is acknowledged, not ignored: We interpret the pooled mean alongside a prediction interval (13.014–55.552 mg/L), which explicitly shows the range of expected values across real-world settings. This is a best practice in environmental meta-analysis (Viechtbauer, 2010; Riley et al., 2011) when heterogeneity is high and subgroups are underpowered.
3. Transparent limitations: In the revised Discussion (Section 4.4) and Conclusion, we now explicitly caution against overgeneralising the pooled mean. We frame it as a contextual indicator, not a one-size-fits-all threshold. We also highlight the need for future meta-analyses to stratify by water type, climate, or fertiliser formulation—once more standardised datasets are available.
4. Data extraction constraints: Of the 18 studies included, fewer than half provided sufficient metadata to categorise by both ecosystem type and fertiliser regime. Several reported only nitrate concentrations without disaggregated values by source or sampling season. This is a common limitation in environmental meta-research, as noted by Borenstein et al. (2019) and Ioannidis (2008), and we address it in Section 4.6 as a recommendation for future research design.

While subgroup analyses would indeed enhance interpretability, the current synthesis still offers practical value: it confirms that nitrate levels associated with synthetic nitrogen fertiliser use routinely exceed U.S. EPA thresholds and, in some cases, approach or exceed WHO and EU limits. This has clear implications for regulatory attention and agricultural nutrient management, even if regional variability remains a caveat.

Reviewer Comment:“The analysis lacks a control group (e.g., non-agricultural or pre-fertilization water quality data), relying solely on nitrate concentrations from fertilized sites. This design precludes causal attribution to fertilizers alone.”

Response: We thank the reviewer for raising this important methodological issue. We fully acknowledge that the absence of control or baseline nitrate concentration data in the included studies limits the ability to draw definitive causal inferences about fertiliser use alone as the sole contributor to observed nitrate levels.

However, this limitation reflects a structural characteristic of the primary environmental literature, not an oversight in this review. Across the 18 studies included, very few reported pre-fertilisation nitrate levels, non-agricultural reference sites, or untreated comparators. As a result, the meta-analytic synthesis necessarily relied on single-group observational data—a common constraint in environmental science, where randomised or quasi-experimental designs are rare due to logistical, ethical, and spatial constraints (see Gutsmiedl et al., 2020; Riley et al., 2011).

To address this, we adopted a single-group meta-analysis model using continuous outcome data (i.e., nitrate concentrations) and framed our conclusions accordingly. We have also revised the Limitations (Section 4.5) and Discussion (Section 4.4) to explicitly acknowledge that our findings demonstrate a strong association—but not isolated causality—between fertiliser use and nitrate levels. Additionally, we highlight the role of potential co-contributors such as land use change, stormwater runoff, and legacy nitrogen accumulation, which may confound nitrate outcomes in agricultural catchments.

Despite the absence of a control group, the convergence of elevated nitrate concentrations across a wide range of fertilised agricultural contexts—combined with well-established mechanistic understanding of nitrogen leaching—supports the interpretation that synthetic nitrogen fertilisers are a major contributing factor, even if not the sole one. Our goal was not to isolate causality but to provide a pooled estimate of exposure in real-world fertilised systems, which has direct relevance for policymaking and risk assessment.

We hope this clarification addresses the reviewer’s concern, and we agree that future meta-analyses would benefit from greater inclusion of comparative or longitudinal study designs where available.

Reviewer Comment:“Funnel plot asymmetry suggests potential publication bias, yet the fail-safe N test (35,906 "missing" studies) is overrelied upon to dismiss bias. This test assumes missing studies are null-finding, which is unrealistic. It is recommended to supplement with Egger’s regression test for bias, then discuss how bias might affect results (e.g., small-study effects exaggerating pollution levels).”

Response: We thank the reviewer for this important observation regarding the limitations of the classic fail-safe N and the need for a more nuanced interpretation of publication bias. We agree that relying solely on the fail-safe N test—particularly given its assumption that all missing studies report null effects—can lead to an overly optimistic conclusion. To strengthen our bias assessment, we have made the following revisions:

1. Egger’s regression test has now been performed and included in the Results section (Section 3.4.6). The regression intercept was 2.87 (SE = 1.21, p = 0.026), indicating statistically significant funnel plot asymmetry and suggesting the presence of small-study effects. We have added this to the manuscript and interpreted it alongside the existing Begg and Mazumdar test, Duval and Tweedie’s trim-and-fill, and funnel plot inspection.
2. We have updated the Discussion section (4.5 Limitations) to reflect the possibility that smaller studies with disproportionately high nitrate concentrations may have contributed to upward bias in the pooled estimate. This concern is now addressed explicitly under the paragraph discussing publication bias.
3. We have revised the interpretation of the fail-safe N test to clarify that while its large value (35,906) suggests robustness, it does not fully eliminate the risk of publication bias and should be understood as part of a multi-tool assessment, not a conclusive standalone metric.

Reviewer Comment: The discussion does not adequately address why nitrate levels vary widely (e.g., soil permeability, rainfall, fertilizer application timing). Regulatory thresholds (WHO/EU limits) are mentioned, but not contextualized against regional variability. I suggest expanding the "Understanding Heterogeneity" section (4.4) to link factors like soil type or climate to observed disparities. In addition, pooled nitrate levels should be compared to local standards (e.g., the EU’s 50 mg/L limit) to assess real-world risks.

Response: Thank you for this important and insightful comment. We agree that the Discussion section would benefit from a more detailed exploration of the environmental and agronomic factors contributing to variability in nitrate concentrations, as well as a clearer contextualisation of regulatory thresholds. In response, we have revised Section 4.4 ("Understanding Heterogeneity") to more explicitly link observed disparities in nitrate levels to site-specific factors such as soil permeability, rainfall patterns, and fertiliser application timing.

We have also strengthened our discussion of how local environmental conditions—such as sandy soils or intensive irrigation practices—can either exacerbate or mitigate nitrate leaching, drawing on specific examples from the included studies. Furthermore, the section now includes a more direct comparison of our pooled nitrate concentrations with regional standards, including the EU limit of 50 mg/L NO₃⁻ and the USEPA threshold, to better illustrate real-world implications. We believe these revisions provide a more nuanced and regionally relevant synthesis of the findings.

Reviewer Comment: On Table 1: Clarify units (e.g., "Mean (mg/L NO₃⁻)")

Response: Thank you for your observation. We have revised Table 1 to clearly indicate the units of measurement. The column heading has been updated to read “Mean (mg/L NO₃⁻)” to specify that the values represent nitrate concentrations in milligrams per litre.

Reviewer Comment: The Z-value (13.457) is misstated as "2-Value" in Figure 3. Correct typographical errors.

Response: Thank you for pointing this out. We would like to clarify that the figure displays Z-values, not "2-Value" as mentioned. However, we acknowledge that the original version may have appeared unclear. To address this, we have revised Figure 3 to improve its overall clarity, including clearer labelling of Z-values, and improved layout. We believe the updated figure resolves any potential confusion and significantly enhances readability.

Reviewer Comment: Figure 5 is cut off. Ensure all panels are visible.

Response: Thank you for your comment. We would like to clarify that Figure 5 is a complete image representing the sensitivity analysis using the “one-study removed” approach. The appearance of a cutoff may stem from the layout of the figure, which includes a central Mean (95% Confidence Level) scale ranging from approximately –350 to +350. Upon closer inspection, all panels and data points are visible within this scale.

Reviewer comment: Briefly cite evidence for precision agriculture’s efficacy (e.g., % reduction in leaching from CRFs or deep placement)

Response: Thank you for the helpful suggestion. In response, we have expanded the discussion in Section 4.3 to include brief, quantified evidence of the efficacy of precision agriculture techniques.

Reviewer comment: Specify which cancers/reproductive disorders are linked to nitrates (e.g., colorectal cancer) for clarity.

Response: Thank you for your suggestion. In response, we have clarified the health section to specify the types of conditions linked to chronic nitrate exposure.

Reviewer comment: Simplify phrasing in entire manuscript (e.g., "synthesised using a single-group meta-analysis of means").

Response: Thank you for this helpful observation. We have carefully reviewed the manuscript and simplified overly technical or complex phrasing where appropriate.

Reviewer comment: Appendix tables (A1–A3) are disorganized and Consolidate and clarify keyword iterations.

Response: Thank you for highlighting this issue. We agree that the original appendix tables (A1–A3) appeared disorganised and may have contributed to confusion regarding keyword iterations. In response, we have consolidated and reformatted the tables to improve structure, clarity, and flow.

These changes ensure a more balanced and transparent discussion of potential publication bias. While we believe our central conclusion—that synthetic nitrogen fertiliser use is strongly associated with elevated nitrate levels—remains well supported, we now present this finding with appropriate caution, particularly in light of possible small-study effects.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors corrected the Manuscript according ro reviewers suggestions.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors thoughtfully addressed my comments. The manuscript can be published in Pollutants.